1. Field of the Invention
The present invention relates to a converging sheet used for illuminating a liquid crystal display, to a surface light source unit, and to a transmission type display using the converging sheet and the surface light source unit.
2. Background Art
A variety of surface light source units used to illuminate transmission type liquid crystal displays or the like from their rear have been proposed and put into practical use. These surface light source units are broadly classified into the edge light type and the direct type, according to the mode of conversion of a non-surface light source to a surface light source.
For example, in a surface light source unit of a direct type, light is introduced from its rear side by means of arc tubes arranged in parallel. The arc tubes and an LCD panel are positioned with a suitable space therebetween. In this space, there are placed a diffusing sheet and a combination of two or more sheets for converging light.
Such a conventional surface light source of a direct type is poor in light-converging properties although a large number of optical sheets are needed. In order to solve this problem, the structure of the LCD panel is improved so that even light that is obliquely incident on the display can produce an image of excellent qualities.
However, this conventional way of improvement causes reduction in optical efficiency, and complicates the structure of the LCD panel, which results in increase in costs.
In particular, the surface light source unit of direct type has a disadvantage in that the light intensity (luminance) on an LCD panel tends to be non-uniform depending on the distance from the arc tubes (i.e., whether a certain point on the LCD panel is close to the arc tube or to the space between the arc tubes arranged in parallel). A possible method of avoiding the non-uniformity is to increase a space between the arc tubes and the LCD panel. However, this method undesirably increases a thickness of the display. Another possible method of avoiding the non-uniformity is to increase a degree of diffusion or to control the amount of transmitting light. However, this method is disadvantageous in that the amount of usable light is decreased.
Specifically, Japanese Patent Laid-Open Publications No. 119703/1993 and No. 242219/1999, for example, disclose a surface light source provided with a light-shielding member (e.g., a lighting curtain or a light-shielding dot layer) so as to maintain the uniformity in light intensity. This method has been disadvantageous in that the amount of usable light is decreased, as stated above.
Japanese Patent Laid-Open Publication No. 347613/1994 proposes use of a sheet having lenticular lenses disposed on both surfaces. However, this structure is not intended to converge light, but intended to control a diffusion of light in two directions. Thus, optical axis on every area of the LCD panel changes according to the position relative to the arc tubes. Therefore, this method has been disadvantageous in that the brightness of light varies depending on the position from which the display screen is observed.